Hybrid supercapacitors (HSCs), combining the advantages of the high specific power of electric double-layer capacitive (EDLC) and the high specific energy of rechargeable batteries, attract extensive interest in the development and applications of modern electronic devices [1-3]. As the core part of supercapacitors (SCs), the electrochemical properties of electrode materials have a huge impact for commercial use. Typically, the energy storage mechanisms of electrode materials may be divided into two categories, include EDLC process and faradaic redox reaction, considerable efforts have been dedicated to the faradaic redox reaction electrodes to achieve higher energy density than EDLC materials [4, 5]. Due to the ultrahigh specific capacities and low costs, transition-metal-based compounds have been widely applied as battery-type electrode materials for HSCs [6]. So far, transition-metal-based oxides/hydroxide including Ni(Co)O [7-10], Co₃O₄ [11-13], Co(Ni)MoO₄ [14-16], NiCo-LDH [17-20], etc. with various crystallographic structures were explored mainly as HSCs electrode materials. However, the low rate performance and inferior stability of these metal oxides/hydroxides were still hindered their potential usefulness because of their poor electrical conductivity, unstable structure and sluggish reaction kinetics [21-23]. Nanomaterials with small sizes and large surface areas, which provide enriched redox reaction sites and superior electronic conductivities, can significantly improve the electrochemical property [24, 25]. Therefore, it is necessary to develop novel nanomaterials with excellent conductivity and stability to overcome the shortfalls of transition metal oxides/hydroxides for improved electrochemical performance.

Transitional metal selenides (TMS) with high electrical conductivity, tunable electronic configuration and electrochemical activity [26, 27], have been regarded as a suitable electrode material for SCs. Up-to-now, TMS electrode materials, such as Ni₃Se₂ [28, 29], NiSe₂ [30, 31], NiSe [32], CoSe [26] and NiCoSe₂ [33-35], have been investigated for HSCs applications. Especially, Ni₃Se₂ is attracting attention in the field of HSCs due to its higher capability than Ni_1-xSe and NiSe₂ [28]. To make the most of the advantages of Ni₃Se₂ as battery-type materials for HSCs, nanoarchitecture engineering has been used to design high-performance nickel-based selenium compounds electrode materials for HSCs, such as mesoporous nanosheets [29], nanowires [36] and nano-dendrite arrays [28], have been explored as battery-type electrode materials for HSCs. At present, the in-situ growth of active nanomaterials on conductive substrates (such as Ni foam, NF) is an attractive approach and has been widespread applied in energy storage devices for outstanding performances. For instance, Chen et al. [28] prepared hierarchical Ni₃Se₂ nano-dendrite arrays on NF, which exhibited a high specific capacitance of 1234 F/g (3.70 F/cm²) at 1 A/g and outstanding rate capability. However, the nano-dendrite Ni₃Se₂ electrode materials suffered from poor electrochemical stability because of their unstable structure, which need further improvement. Furthermore, Wang et al. [36] also synthesized Ni₃Se₂ rich-grain-boundary nanowire arrays on NF by a solvothermal/selenization process. The Ni₃Se₂ nanowire arrays produced a high areal capacity (635 mAh/cm² at 3 mA/cm²) and superior rate capability. Unfortunately, the approach required fussy, time-consuming and high-temperature processes, which posed a big challenge for scaling-up production. Thus, a simple and green method for large-scale production of Ni₃Se₂ nanomaterials with efficient and robust capacitor performance should be proposed and designed.

Herein, we rationally designed a hierarchical nanoarchitecture based on Ni₃Se₂ nanosheet-on-nanorods core-shell structure electrode materials (NSR_x-Ni₃Se₂, x represents 0.5, 0.8 and 1.0) via a simple 3D NF-assisted solvothermal strategy. This freestanding 3D nanoarchitecture enhanced the contact area with the electrolyte and provided fast electron and ion transport channels, thus significantly improve the electrochemical performance. As a result, it exhibited a high specific capacity of 1.068 mAh/cm² (7.69 F/cm²) at 2 mA/cm² and an excellent rate performance. Furthermore, we assembled a HSC device based on the NSR_0.8-Ni₃Se₂, which showed a fantastic energy density of 56.4 Wh/kg at 386.5 W/kg, an outstanding power density of 4640.3 W/kg at 39.7 Wh/kg and superior cycling performance (92.6% retention after 6000 cycles).

The novel NSR_x-Ni₃Se₂ was fabricated via a simple 3D NF-assisted solvothermal strategy as shown in Fig. 1. The NF was immersed in a seed solution containing selenium (Se), where it underwent solvothermal treatment to initiate the in-situ formation of the Ni₃Se₂ nanosheet-on-nanorods core-shell structure. By changing the addition of Se (ranging from 0.5, 0.8, 1.0 mmol), the morphology of NSR-Ni₃Se₂ can be dramatically tuned (Fig. S1 in Supporting information), which were named as NSR_0.5-Ni₃Se₂, NSR_0.8-Ni₃Se₂, NSR_1.0-Ni₃Se₂, respectively. When the amount of Se powder was 0.8 mmol, homogeneous Ni₃Se₂ arrays were in-situ grown on the NF (Figs. 2a and b, Figs. S1a and b). The NSR_0.8-Ni₃Se₂ has an average diameter of ca. 400 nm, which showed nanosheet-on-nanorods core-shell structures (Figs. 2c and S2 in Supporting information). The corresponding elemental mapping images (Figs. 2d and e) revealed that the coexistence and uniform distribution of Ni and Se in a single NSR_0.8-Ni₃Se₂. The high-resolution transmission electron microscopy (TEM) of the NSR_0.8-Ni₃Se₂ showed lattice fringes with spacings of 0.212 nm and 0.301 nm, assigning to the (202) and (110) planes of Ni₃Se₂ respectively (Fig. 2f). This novel homogeneous nanostructure not only offers abundant energy storage active sites, but also promotes high-speed electron transfer, which can hugely improve the electrochemical performance. The crystal phase of the obtained NSR_x-Ni₃Se₂ samples were characterized by XRD patterns (Fig. 2g). Except for the diffraction peaks at 44.50°, 51.80° and 76.37° for NF (PDF#04-0805), the diffraction peaks at 2θ angle of 20.94°, 29.58°, 29.97°, 37.17°, 42.62°, 47.68° and 52.73° corresponding to (101), (110), (012), (003), (202), (211) and (122) planes of Ni₃Se₂ (PDF#85-0754) respectively, are clearly appeared in all the three NSR-Ni₃Se₂ samples. X-ray photoelectron spectroscopy (XPS) spectrum was then employed to further confirm the chemical composition of the NSR-Ni₃Se₂ samples. The high-resolution Ni 2p spectrum (Fig. 2h) of NSR_0.8-Ni₃Se₂ showed two major peaks at 855.6 eV (Ni 2p_3/2) and 873.3 eV (Ni 2p_1/2), which could be indexed to Ni²⁺ [36]. The other two small peaks at 861.5 eV and 879.5 eV belong to the shake-up satellites. The Se 3d XPS spectrum for the NSR_0.8-Ni₃Se₂ (Fig. 2i) contains two peaks at 55.2 eV and 56.1 eV, corresponding to the metallic Se 3d and sulfur-metal bonds, respectively [33]. The above results indicated the successful formation of Ni₃Se₂ by the 3D NF-assisted solvothermal strategy.

Electrochemical properties of the NSR_x-Ni₃Se₂ samples were firstly analyzed by cyclic voltammetry (CV) in a three-electrode system at 2 mV/s (Fig. 3a). Compared to CV of NSR_0.5-Ni₃Se₂ and NSR_1.0-Ni₃Se₂, the voltammetric current response of NSR_0.8-Ni₃Se₂ was much larger, implying the capacity of NSR_0.8-Ni₃Se₂ was much higher than the other two counterparts. The redox reaction mechanism of Ni₃Se₂ can be described as the following equations in the KOH electrolyte:

The CV curves of the NSR_0.8-Ni₃Se₂ were also displayed from 2 mV/s to 20 mV/s with the increasing of the scan rates. As shown in Fig. 3b, the response current of the NSR_0.8-Ni₃Se₂ rose linearly as scan rate increases and the CV plot shapes was highly stable, indicating the highly reversibility and ultrafast ion/charge transport kinetics of the NSR_0.8-Ni₃Se₂. The reaction kinetics was further probed by analyzing the relationship between peak current (i) and scan rate (v) according to the following equations [37, 38]: i = av^b, where a and b are constants. The b-values for the NSR_0.8-Ni₃Se₂ were 0.58 and 0.52 (Fig. S3e in Supporting information), respectively, which indicated that the redox process in the NSR_0.8-Ni₃Se₂ electrode material was dominated by a diffusion-controlled battery-type behavior [14]. Galvanostatic charge/discharge (GCD) analysis of the NSR_x-Ni₃Se₂ was also evaluated at 2 mA/cm² as shown in Fig. 3c. The battery-type of the two GCD profiles, namely latent voltage plateaus, confirmed that the faradaic reduction reactions were occurred during the charge-discharge processes, in good agreement with the aforementioned CV results. As a result, the charge-discharge time of the NSR_0.8-Ni₃Se₂ electrode (3912 s) was much longer than that of the NSR_0.5-Ni₃Se₂ electrode (2262 s) and the NSR_1.0-Ni₃Se₂ electrode (3224 s, Fig. S3 in Supporting information), which could be attributed to its higher specific capacity of NSR_0.8-Ni₃Se. The GCD curves of the NSR_0.8-Ni₃Se₂ (Fig. 3d) showed well defined potential plateaus and relatively symmetric shape at all the current densities, demonstrating its good battery-type property and high reversibility. Based on the GCD curves results, the areal specific capacity at different current densities of the NSR_x-Ni₃Se₂ samples could be calculated as shown in Fig. 3e. The areal specific capacity values of the NSR_0.8-Ni₃Se₂ were about 1.068 (7.69), 1.006 (7.24), 0.942 (6.78), 0.887 (6.31), 0.847 (6.10), 0.797 (5.74), 0.764 (5.50) and 0.729 (5.25) mAh/cm² (F/cm²) at 2, 3, 5, 8, 10, 15, 20 and 30 mA/cm², respectively, indicating the 68.3% retention of its initial capacity. In contrast, the capacity retentions of the NSR_1.0-Ni₃Se₂ and the NSR_0.5-Ni₃Se₂ were 64.5% and 60.9%, respectively. Notably, the superior specific capacity of the optimized NSR_0.8-Ni₃Se₂ was highly competitive with those of the most previously reported nickel selenide-based electrodes and nickel-based electrodes (Table S1 in Supporting information).

The reaction kinetics of the NSR_x-Ni₃Se₂ was explored by the electrochemical impedance spectroscopy (EIS, Fig. 3f). The slope of the NSR_0.8-Ni₃Se₂ electrode was steeper than that of the NSR_0.5-Ni₃Se₂ and NSR_1.0-Ni₃Se₂ in the low frequency region, suggesting that the NSR_0.8-Ni₃Se₂ electrode possess a short path for the electrons transportation and ions diffusion. The corresponding ohmic resistances for the NSR_x-Ni₃Se₂ were 0.91 (NSR_1.0-Ni₃Se₂), 0.86 (NSR_0.8-Ni₃Se₂) and 0.81 (NSR_0.5-Ni₃Se₂) Ω/cm², respectively. This result suggests that the three NSR_x-Ni₃Se₂ samples each have high electrical conductivity, while the NSR_0.8-Ni₃Se₂ possesses the best electrochemical activity, which in turn indicates that the nanosheet-on-nanorods core-shell structure is very important to the performance. On the above basis, the significant enhancement in the capacitor performance of our 3D NF-assisted solvothermal strategy-derived NSR_0.8-Ni₃Se₂ sample can be explained by the following reasons: (1) The in-situ preparation strategy guarantees the freestanding structure and robust support of electroactive materials of the hierarchical NSR_x-Ni₃Se₂ on NF, which can significantly improve specific capacity and cycling stability. (2) The in-situ growth also reduces the interface resistance gap between the current collectors and electroactive materials, and acts as an electron superhighway to enhance the ion/electron transfer rate. (3) The hierarchical characteristic can offer a high specific surface area and plenty of active sites to store electrolyte ions, where the one-dimensional nanorod skeleton cannot only act as a high-speed electron transfer channel, but also can avoid the aggregation of the nanosheets. Additionally, the two-dimensional nanosheets provide numerous exposed active edge sites and protect the backbone from electrochemical corrosion.

The electrochemical properties of the NSR_0.8-Ni₃Se₂ materials for real application were also investigated by using two-electrode HSCs device, in which the NSR_0.8-Ni₃Se₂ electrode materials were used as the cathode, active carbon (AC) was used as the anode, and a porous glassy fibrous paper was used as the separator. The NSR_0.8-Ni₃Se₂/AC mass ratio is about 0.28 according to the equation: m⁺/m⁻ = C⁻ΔE⁻/(C⁺ΔE⁺) [39]. To obtain the maximum capacity and proper voltage range for the NSR_0.8-Ni₃Se₂//AC device, CV curves were tested at different voltage windows ranging from 0.0-1.1 V to 0.0–1.7 V. As shown in Fig. 4a, no apparent polarization even at the voltage window of up to 1.6 V was observed, suggesting that 0.0–1.6 V was an apropos voltage window for the NSR_0.8-Ni₃Se₂//AC device. Fig. 4b showed that the NSR_0.8-Ni₃Se₂//AC device had a superior stability over the voltage range of 0.0 V to 1.6 V, and no obvious distortion of the CV curves as scan rate increased, signifying that the fast and stable electron transfer kinetics of the as-assembled device. Furthermore, the GCD curves of the NSR_0.8-Ni₃Se₂//AC device (Fig. 4c) were quasi-triangular shape with symmetric charge/discharge time, proving its excellent reversibility. Its areal specific capacity (Fig. 4d) reached 0.88 mAh/cm² at 2 mA/cm² that could maintain as 0.62 mAh/cm² (70.4% retention of the initial capacity), meanwhile its coulombic efficiency was nearly 100% at 30 mA/cm². Moreover, our HSCs device delivered an ultrahigh energy density of 56.4 Wh/kg at 386.5 W/kg, and the energy density could still remain 39.7 Wh/kg at 4640.3 W/kg (Fig. 4e). Compared with the previously reported nickel selenides-based electrodes [28, 29, 33, 34, 36, 40], our HSCs device shows an ultrahigh energy and power densities (Table S2 in Supporting information). The cycling stability of the HSC device was further explored at 30 mA/cm² (Fig. 4f). It can be seen that the capacity retention is as high as 92.6% after 6000 cycles, accompanied by almost 100% coulombic efficiency, confirming that the excellent stability with high coulombic efficiency. Impressively, by assembling two HSCs devices in series, three red LEDs (the operating voltage and power is 2.0 V and 30 mW) can be easily lighted up, demonstrating the viability and potential of the HSCs device for practical applications.

In summary, the hierarchical NSR-Ni₃Se₂ core-shell nanoarrays were designed as freestanding electrodes by a simple NF-assisted confinement assembly method, which presented excellent electrochemical performance for the HSCs device. The superior electrochemical performance was ascribed to the novel nanosheets wrapped nanorods core-shell architecture with significantly improved electroactive sites, the 3D network architecture with fast electron transfer channels, and the obviously enhanced contact area with the electrolyte. Our work not only developed a novel and efficient battery-type material, but also provided a simple approach to design 3D hierarchical nanostructures for energy storage devices.

The authors declare no conflict of interest.

We acknowledge the financial support from the National Key R & D Program of China (Nos. 2017YFB1104300 and 2016YFA0200200) and National Natural Science Foundation of China (Nos. 21575014, 21905025, 91963113).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.06.021